Vacuum plasma processing chambers are employed for processing workpieces, typically glass, semiconductor or metal substrates, so the workpiece is etched and/or has materials deposited thereon. Significant amounts of power must be supplied to the vacuum plasma processing chamber to excite ions in a gas in the chamber to a plasma and to maintain the plasma in a discharge state.
The impedance of a load of the vacuum plasma processing chamber, including the plasma discharge and an excitation circuit component for it, usually a coil or one or more electrodes, frequently varies, non-linearly and unpredictably. For example, the quality factor (Q) of an excitation coil prior to ignition of the plasma is typically quite high, about 1,000. After ignition, the Q of the coil and its plasma load drops to about 10-20 when the coil is not close to being resonated. As resonance is approached, the Q of the coil and plasma drops to a lower value because the resistive impedance seen looking into a matching network driving the coil and plasma becomes a larger percentage of the total impedance seen looking into the matching network. The reactive impedance seen looking into the matching network also decreases as resonance is approached and reached due to properties of the plasma. The resistive impedance results, to a large extent, from the charged carriers in the plasma. Hence, matching to a plasma presents particular problems due to these variations in the resistive (real) and reactive (imaginary) impedance components of the load seen looking into the matching network driving the excitation circuit element and plasma of a vacuum plasma processing chamber. These variations often include fairly high frequency components, for example, in the audio frequency range.
Usually the plasma of a vacuum plasma processing chamber has been excited to a single r.f. frequency, such as 13.56 mHz, by a relatively high power r.f. source connected to the load via a variable impedance matching network. Two or more variable impedances of the matching network have usually been provided, one primarily controlling the load reactive component and the other primarily controlling the load real, i.e., resistive, component, as seen by the source.
The variable impedance elements of the matching network must be relatively robust because relatively large r.f. currents and voltages are applied to them. Hence, the variable reactances of the matching network are usually controlled by servo controllers including motors having relatively slow response times, incapable of following many changes in the reactive impedance of the load including the discharge. The prior art controller time constants are typically in the 0.1 to 1 second range, to prevent the matching network variable reactances from adjusting properly to load transients or perturbations which occur at audio frequencies on faster time scales than 0.1 to 1 second. This problem is particularly acute because of the frequent need to repeatedly vary both reactances of the matching network.
It is, accordingly, an object of the present invention to provide a new and improved apparatus for and method of controlling power supplied to a plasma discharge of a vacuum plasma processing chamber.
Another object of the invention is to provide a new and improved method of and apparatus for substantially eliminating reactive components seen by a source exciting a plasma discharge of a vacuum plasma processing chamber.
A further object of the invention is to provide a new and improved method of and apparatus for varying a parameter of an excitation circuit for a vacuum plasma processing chamber in such a manner that reactive impedances seen by a source exciting a plasma discharge of the chamber are substantially eliminated, even when changes in the reactive impedance occur at audio frequencies.
An additional object of the invention is to provide a new and improved all-electronic, low power method of and apparatus for controlling the reactive impedance seen by a source exciting a plasma discharge of a vacuum plasma processing chamber.
Yet another object of the invention is to provide a new and improved method of and apparatus for controlling the amount of power exciting a plasma discharge of a vacuum plasma processing chamber while substantially eliminating reactive impedance effects seen by a source exciting the plasma discharge.
Still a further object of the invention is to provide a new and improved method of and apparatus for controlling the amount of power exciting a plasma discharge of vacuum plasma processing chamber so a desired power level is applied to the load even though power from the load is reflected back to an excitation power supply.